Magnetic recording media have found a variety of applications such as audio tapes, video tapes, computer tapes, magnetic discs, magnetic cards, etc. Particularly in the field of data backup tapes, magnetic tapes having a memory capacity of several hundred GB or more per reel have been commercialized in association with the tendency of the mass storages of hard discs for backup, and a mass storage backup tape having a memory capacity of exceeding 1 TB has been proposed. Under these circumstances, magnetic recording media having far higher density recording performance will be indispensable in future.
In the manufacturing of magnetic tapes capable of corresponding to such high density recording, highly advanced techniques are employed to manufacture fine magnetic powder (hereinafter also referred to as magnetic particles), to fill coating layers with such magnetic powder at higher densities, to smoothen coating layers and to form thinner magnetic layers.
Regarding the improvement of the magnetic powder, trials to reduce the sizes of magnetic particles and simultaneously improve the magnetic characteristics thereof have been made in order to record signals with shorter wavelengths. For this improvement, needle-shaped metallic magnetic particles having an average particle size of 25 to 65 nm (JP-A-2004-79004), plate-shaped hexagonal ferrite magnetic particles having an average particle size of 50 nm or less (JP-A-2004-30828), and spherical or ellipsoidal rare earth element-iron-boron magnetic particles having an average particle size of 50 nm or less (JP-A-2001-181754) are proposed. To prevent a decrease in output from a magnetic tape due to demagnetization in association with the recording of signals with short wavelengths, trials to manufacture magnetic recording media having higher coercive forces have been more vigorously made in these years.
On the other hand, the improvement of the manufacturing technology for magnetic recording media confronts some difficulties. In association with the high density recording of data on magnetic media, the wavelengths of signals of data to be recorded become shorter and shorter. When the magnetic layer of a magnetic medium is thick, the influences of loss due to self demagnetization during the recording/reproducing of data and loss due to the thickness of the magnetic layer increase and then the decrease of output become more serious, although such problems hitherto have not been so seriously taken. Therefore, the reduction of the thickness of the magnetic layers of magnetic media is needed.
However, when the thickness of a magnetic layer is reduced, there arise some problems such that a leakage magnetic flux from a magnetic layer is weakened, and that the surface roughness of a non-magnetic substrate is greatly reflected on the surface of the magnetic layer and degrades the surface smoothness of the magnetic layer. When the thickness of a magnetic layer is reduced, a method of decreasing the solid content in a magnetic coating composition or a method of decreasing the amount of the magnetic coating composition to be applied is considered. However, these methods are not effective to eliminate the defects in a coating step or to increase the amount of magnetic particles filled in the magnetic layer, which leads to the lower strength of the coating layer.
For the above reason, to reduce the thickness of a magnetic layer by improving the manufacturing technology for media, a so-called concurrent layer-superposing system is proposed. In this system, a non-magnetic layer (hereinafter referred to as a non-magnetic primer layer, a primer layer or a lower layer) is provided between a non-magnetic substrate and a magnetic layer, and an upper magnetic layer is applied while the non-magnetic primer layer is still being wet (cf. JP-A-2004-79004).
To cope with the weakening of the leakage flux from the magnetic layer, highly sensitive magnetoresistance (MR) heads are dominantly used as reproducing heads in the systems using such magnetic recording media. The MR heads have no induction coil and therefore cause less mechanical noises, which leads to a higher C/N ratio, since noises from the magnetic recording media can be lessened.
However, the MR heads have a disadvantage such that noise, i.e. thermal noise, tends to be generated by the collision of the minute unevenness of the surfaces of magnetic layers to the MR device, while such minute unevenness have caused few problems in the magnetic induction type heads. Therefore, more careful attentions are needed to control the surface roughness of the magnetic layers.
As described in the above, since a magnetic layer is designed to be very thin, the shapes of the surface roughness thereof is greatly influenced by the particle size of a non-magnetic powder contained in the magnetic layer. The relationship of the thickness of the magnetic layer with the particle size of the non-magnetic powder contained in the magnetic layer and/or a primer layer have been repeatedly studied (cf. JP-A-5-197946, JP-A-7-272256, JP-A-2004-5896 and JP-A-8-263829).
In order to provide a magnetic recording medium having excellent high density recording performances and also good durability, it is necessary (1) to use a fine particle magnetic powder, (2) to decrease the thickness of a magnetic layer and (3) to suitably control the surface condition of the magnetic layer.
JP-A-2004-79004, JP-A-2004-30828, JP-A-2001-181754 and JP-A-2004-5896 disclose magnetic recording media comprising a fine particle magnetic powder. However, they do not satisfactorily meet the above requirements (1), (2) and (3), and cannot sufficiently achieve the high recording density and durability at the same time.
JP-A-5-197946 and JP-A-7-272256 disclose the improvement of the durability of magnetic recording media by controlling the average particle size of a non-magnetic powder contained in a magnetic layer. However, they cannot achieve sufficient high recording density performances, since the particle size of a magnetic powder is too large or the thickness of the magnetic layer is too large.
JP-A-8-263829 discloses a magnetic recording medium comprising a magnetic layer and a layer containing an abrasive, which is provided adjacent to the magnetic layer, in which a ratio of the average particle size of the abrasive to the dry thickness of the magnetic layer is 1.0:1 to 5.0:1. However, it cannot achieve sufficiently high recording density performances, since the particle size of a magnetic powder is too large or the shape anisotropy of the magnetic powder is too large.
Accordingly, the magnetic recording media of the above patent applications cannot have a high capacity and good durability.